1. Technical Field
This invention relates to the art of AM radio broadcasting and, more particularly, to an RF power amplifier system having an improved power supply.
2. Description of the Prior Art
The U.S. Patent to H. I. Swanson U.S. Pat. No. 4,580,111 discloses an amplitude modulator for use in AM radio broadcasting wherein the modulator serves to generate amplitude modulated signals by selectively turning on and off a plurality of RF amplifier modules to produce amplitude modulation. The amplifier modules include big step or unit step modules that each, when on, provide an amplified voltage based on the operating voltage supplied to the amplifier. This is a DC voltage obtained from rectifying a three phase AC input voltage. This big step or operating voltage may be referred to as a B+ voltage. Some of the amplifier modules in such a system are referred to as binary or fractional step amplifier modules, each when turned on providing an amplified output signal having a value which is a fraction of the unit or big step voltage.
In prior systems, the operating voltage for the binary amplifier modules was obtained from a binary power supply which utilized a separate single phase secondary winding tapped from the main three phase power transformer with suitable rectifying circuitry to create either a B+/2 or B+/4 operating voltage for the binary amplifier module. Some difficulty with such power supplies has been encountered. There is no adjustment of the DC voltage provided by the binary amplifier power supply and typically there is a tolerance of + or -5%. This has created an error in the fractional step level when compared with a full or big step. If a big step is on the order of 250V, then 1/2 step is expected to be on the order of 125V. A 5% error may result in a deviation of + or -10V from this anticipated 1/2 step level. It is desirable to reduce this error. External correction methods have been employed to adjust the size of the binary step relative to the big step. The methods employed have only allowed coarse adjustments and have not been able to remove all the error introduced by the windings of the power supply.